1. Technical Field of the Invention
The present invention relates to a position detector, and more particularly to a position detector using a magnetic scale member.
2. Description of the Related Art
A magnetic scale sensing apparatus using a position detector (magnetic scale), which includes a magnetic sensor using a magnetoresistance (MR) element or the like and a scale member which is magnetically patterned, has been known as an origin detection device of a magnetic encoder.
To determine the position of the magnetic sensor, the magnetic scale sensing apparatus needs to detect the position of an origin of the scale. In one known origin detection method, a magnetic mark for origin detection is provided in a channel different from that of the scale member, and the origin is determined by detecting the magnetic mark (for example, see Patent Document 1). In the conventional method of detecting the origin point, a magnetic mark for origin point detection is provided on a separate channel, hence a magnetic sensor for detecting the origin point must be provided separately from another magnetic sensor for scaling.
The magnetic scale sensing is applied, for example, to a slide control used for setting parameters corresponding to filter characteristics or frequency characteristics of input and output signals, or parameters for adjusting input and output levels in a mixing console (for example, see Patent Document 2).
When the magnetic scale sensing is applied to a slide control, for example, a magnetic sensor is mounted on a moving block and a magnetic pattern is detected as the moving block is moved in a movement guide carrying a magnetic scale member having the magnetic pattern. The position of the moving block, the amount of movement, and the direction of movement thereof are determined based on the detection of the magnetic pattern, and parameters are set based on the determination.
The magnetic scale sensing can achieve highly accurate detection while the position detector decreases in size. However, difficulties in managing the gap between the magnetic scale and the magnetic sensor are on the rise. The gap between the magnetic scale and the magnetic sensor that is mounted movable relative to the magnetic scale must decrease as the pitch (interval) between poles of the magnetic scale decreases.
[Patent Document 1] Japanese Examined Patent Application Publication No. 6-84893
[Patent Document 2] Japanese Patent Application Publication No. 2006-332074
[Patent Document 3] Japanese Utility Model Application Publication No. 6-53915
[Patent Document 4] Japanese Patent Application Publication No. 11-148842
[Patent Document 5] Japanese Patent Application Publication No. 2005-195367
[Patent Document 6] Japanese Patent Application Publication No. 2007-227055
In the conventional method of detecting the origin point, a magnetic mark for origin point detection is provided on a separate channel, hence a magnetic sensor for detecting the origin point must be provided separately from another magnetic sensor for scaling, thereby causing problems such as over-size of the position detector which causes enlargement of appliances using the position detector and increase of costs.
Generally, since the position detector is constructed such that the magnetic sensor moves relative to the magnetic scale, the position detector needs clearance (allowance) for movement. Slidability is degraded if the clearance is set to be as small as possible, whereas loss of sensing pulses or jouncing may occur if the clearance is set too large.
Therefore, it is necessary to set appropriate clearance when setting the magnetic scale and the magnetic sensor. However, the gap between the magnetic scale and the magnetic sensor opposing each other may be reduced below a critical level due to a variety of reasons such as uneven clearance within a slide range due to aging. In this case, it is difficult to obtain a resolution of signals having a logic value “0” or “1” when employing a method in which an origin, which is a Z phase representing the reference point of the magnetic scale, and A and B phases, which are signals that are 90 degrees out of phase and are produced by reading pitches magnetically recorded on the scale, are detected together. That is, the logic value may be erroneously kept at “1” or may be kept at “0.”
Taking into consideration these circumstances, one can consider increasing the gap between the magnetic scale and the magnetic sensor and decreasing the magnetic intensity (magnetization) of the magnetic scale in order to perform appropriate sensing. However, these solutions are susceptible to external magnetic disturbance. In addition, the gap between the magnetic scale and the magnetic sensor needs to decrease as the accuracy of the position detector increases and the size thereof decreases as described above. Further, when the magnetic scale is weakly magnetized, the magnetic scale is very susceptible to disturbance such as magnetic fields produced by a magnet located near the magnetic scale, for example, the magnetic scale is easily demagnetized and the magnet of the magnetic scale is weakened or directed sideways due to disturbance. Accordingly, it is necessary to magnetize the magnetic scale in a full range (i.e., to magnetize the magnetic scale so as to maximize the area of a hysteresis loop of magnetization). Furthermore, if the gap between the magnetic scale and the magnetic sensor is simply set to be small and the magnetization of the magnetic scale is simply set to be large, then the magnetic sensor may oppose the magnetic scale with its magnetic sensitivity being within a saturation range and it is thus difficult to obtain sufficient signal resolution.